The present invention relates to a frequency carrier allocating method suitable in the FDD (Frequency Division Duplex) cellular system where bidirectional communication is performed between a base station and mobile stations by using different frequency carriers in up-stream and down-stream. Particularly, the present invention relates to a frequency carrier allocating method suitable in a cellular system that provides multimedia communication services in which the traffic in up-stream is asymmetric with the traffic in down-stream and in which a base station allocates a frequency carrier not used by neighbor cells according to traffic.
In the FDD cellular system, the base station simultaneously performs its down-stream transmitting operation and its up-stream receiving operation using different frequency carriers while the mobile station simultaneously performs its up-stream transmitting operation and its down-stream receiving operation using different frequency carriers. The base station communicates with many mobile stations within a cell using plural frequency carriers. Many frequency carriers can be used within one cell simultaneously and repeatedly using each frequency carrier within plural cells geographically remote from each other.
In the transmitter/receiver shown in FIG. 8, an interference power measurement circuit 830 and plural receiving circuits 840, plural transmission circuits 850 are connected to antenna 810 via the transmit/receive duplexer 820. A mobile station, as shown in FIG. 9, includes a receiving circuit 940 and a transmission circuit 950 which are connected to the antenna 910 via the transmit/receive duplexer 920. The base station uses a transmit/receive duplexer for achieving separation between a transmission signal and a received signal each having a different frequency. The mobile station uses a transmit/receive duplexer for achieving separation between a transmission signal and a receive signal each having a different frequency. However, since the difference in level between the transmission signal and the receive signal is large, the two signals must be sufficiently separated from each other. Hence, it is required that the frequency interval between the frequency group in the up-stream and the frequency group in the down-stream is sufficiently larger than that between frequency carriers used in the same direction.
For that reason, in the FDD system, as shown in FIG. 10, the transmit/receive duplexer uses two frequency bands spaced away by a necessary frequency interval to separate the transmission signal and the receiving signal. One frequency band is allocated only for the up-stream while the other frequency band is allocated only for the down-stream. Hence, the number of carriers usable in each stream is fixed. Normally, two frequency bands are equal in bandwidth. The number of frequency carriers usable in the up-stream is the same as that in the down-stream.
Hence, there has the problem that when the ratio of the width of one frequency band to the width of the other frequency band differs from that of the upper-stream traffic to the down-stream traffic, the frequency band in traffic decreasing direction cannot be sufficiently used. In order to solve such a problem, JP-A-275230/1996 discloses the method of making the frequency carrier pass band in a down-stream larger than that in an up-stream and alternately interchanging the up-stream frequency carrier and the down-stream frequency carrier in each frequency band.
In this method, a frequency carrier in a frequency band which is different in the up-stream and the down-stream is allocated to each mobile station. Thus, the frequency band use efficiency is improved when the traffic in the down-stream is larger than that in the up-stream while the transmission to receive interval necessary for the transmit/receive duplexer is reserved.
In the prior art, the frequency use efficiency is maximized when the ratio of the information amount in the up-stream to the information amount in the down-stream is equal to that of the passage band for the up-stream to the passage band for the down-stream. However, the prior art has the problem in that frequency-carriers are not optimally allocated in the frequency band when the information amount ratio is not equal to the passage band ratio, so that the frequency band use efficiency is decreased. Where it is difficult to predict the ratio of the information amount in the up-stream to the information amount in the down-stream, or the ratio changes during the system operation, or the information transmission efficiency per frequency band changes due to future technical advances, the problem is whether or not the frequency band is adaptively allocated in the up-stream and the down-stream.
The objective of the present invention is to solve the above-mentioned tasks.
The object of the present invention is to provide a frequency carrier allocating method that adaptively allocates frequency carriers in each direction according to a change in the ratio of a traffic in up-stream to a traffic in down-stream while the frequency interval between the frequency carrier in up-stream and the frequency carrier in down-stream is maintained at a fixed value or more in each base station.
Another object of the present invention is to provide a frequency carrier allocating method that can always provide a high frequency use efficiency even if the ratio of a traffic in up-stream to a traffic in down-stream varies.
In order to accomplish the above mentioned objects, the objective of the present invention is achieved by the frequency carrier allocating method for a cellular system, the cellular system including plural base stations and plural mobile stations, the cellular system having two frequency bands each in which plural frequency carriers each divided into plural time slots are arranged and in which frequencies are not overlapped, each of the plural base stations allocating a frequency carrier and a time slot in a down-stream for transmission from a base station to a mobile station according to a communication request from a mobile station and allocating a frequency carrier and a time slot in an up-stream for transmission from a mobile station to a base station according to a communication request from a mobile station, so that the cellular system performs bi-directional communications between the base station and the mobile station, the method comprising the steps of dividing the plural base stations into two groups including a first group and a second group; allocating a frequency carrier used in an up-stream from a first frequency band among the two frequency bands in a base station belonging to the first group, and allocating a frequency carrier used in a down-stream from a second frequency band among the two frequency bands in the base station belonging to the first group; and allocating a frequency carrier used in an up-stream from a second frequency band among the two frequency bands in a base station belonging to the second group, and allocating a frequency carrier used in a down-stream from a first frequency band among the two frequency bands in the base station belonging to the second group.
In the frequency carrier allocating method according to the present invention, the required number of time slots for a frequency carrier are allocated according to a traffic in an up-stream and a traffic in a down-stream.
In the frequency carrier allocating method according to the present invention, the same number of the base stations belonging to the two groups is substantially included in a given group of cells each in which the same frequency carrier cannot be simultaneously used; and each of the base stations are located to be adjacent to more base stations belonging to different group.
In the frequency carrier allocating method according to the present invention, the frequency carrier allocating step comprises the steps of sequentially selecting from one side of the frequency carrier arrangement in each frequency band in a base station belonging to the first group; sequentially selecting from the other side of the frequency carrier arrangement between base stations belonging to the second group; and allocating a frequency carrier first-satisfying a quality condition.
According to the present invention, the frequency carrier controlling method further comprises the step of dynamically allocating a frequency carrier and time slot in accordance with an increase or decrease in the up-stream traffic and the down-stream traffic of each base station.
Base stations are divided in groups in such a manner that among an arbitrary group of cells in which the same frequency carriers cannot be simultaneously used, a half of base stations substantially belongs to one group while base stations as many as possible are located adjacent to base stations belonging to another group.
Moreover, a frequency carrier is dynamically allocated according to the traffic ratio or traffic variation in the up-stream and the down-stream of each base station. In a frequency carrier allocating operation, a base station belonging to the same group selects frequency carriers in the same order in each frequency band and selects frequency carriers in a reverse order between base stations belonging to a different group, so that a frequency carrier first-satisfying quality conditions is allocated.